Numerous devices and methods are known for measurement of soil constituents, but in general this cannot be done in real time when seeding or fertilizing soil. It is known to use an instrumented disc coulter (3 feet, 76 cms, in diameter) using a rotary potentiometer and an ultrasound proximity sensor, as soil mechanical resistance and depth measurement. Basically the proposed invention measures constituents in situ, in vivo, in real time. The measuring system may be connected wirelessly or hard wired via the axle of the coulter to data processing system connected to a computerised control system in the tractor cab. The constituent data can be processed to modify seeding of the soil, or fertilizer deposition in the soil as seeding and fertilizer deposition are performed.